Coal unions have thumbed their noses at a threat by BHP Billiton to withdraw its generous pay offer if the 18-month industrial dispute drags on.

The shock closure of the Norwich Park mine sends a powerful warning to unions that mining giants can shut sites even in a resource boom.

Peabody Energy says it's obtained approval from the Foreign Investment Review Board (FIRB) of its $15.50 per share bid for Macarthur Coal Ltd. Macarthur has advised shareholders to take no action on the offer from Peabody through its subsidiary, PEAMCoal.